For Corals Adapting to Climate Change, It’s Survival of the Fattest—and Most Flexible

Study suggests best targets for environmental conservation

A closeup of polyps of Orbicella faveolata, more commonly known as boulder coral. Image courtesy of The Ohio State University.

COLUMBUS, Ohio—The future health of the world’s coral reefs and the
animals that depend on them relies in part on the ability of one tiny symbiotic
sea creature to get fat—and to be flexible about the type of algae it cooperates
with.

In the first study of its kind, scientists at The Ohio State
University discovered that corals—tiny
reef-forming animals that live
symbiotically with algae—are better able to recover from yearly bouts of heat
stress, called “bleaching,”
when they keep large energy reserves—mostly as fat—socked away in their cells.

“We found that some coral are able to acclimatize to annual
bleaching, while others actually become more susceptible to it over time,” said
Andréa Grottoli, professor in the
School of Earth Sciences at Ohio
State. “We concluded that annual coral bleaching could cause a decline in coral
diversity, and an overall decline of coral reefs worldwide.”

Andréa Grottoli

The study, which appears in the July 9 online edition of Global
Change Biology, indicates that some coral species will almost certainly
decline with global climate change,
while others that exhibit large fat storage and flexibility in the types of
algae they partner with will stand a better chance of enduring repeated rounds
of stress as oceans get hotter.

It also suggests that the most adaptable species would make
good targets for conservation efforts because they are most likely to survive.

“If we conserve reefs that contain coral species with these
survival traits, then we’re hedging our bets that we might be able to preserve
those reefs for an extra decade or two, buying them enough time to acclimatize
to climate change,” Grottoli said.

Corals are essentially colorless; the brilliant browns,
yellows, and greens that we associate with them are actually the colors of
algae living inside the corals’ animal cells. That’s why, when stressed coral
dump most of the algae from their cells, their bodies appear pale, or
“bleached.”

Bleached corals can recover by growing more algae or
acquiring new algae once water temperatures return to normal. This research shows that corals’ ability to
switch the type of algae that they internally grow has a large effect on their
recovery.

But if corals don’t recover and reefs die, thousands of fish
species and other sea creatures lose their habitat.

Normally, bleaching is a rare event. But by 2025, Caribbean
waters are expected to be hot enough that the coral living there will be
stressed to the point of bleaching once a year. The rest of the tropics are
expected to experience annual bleaching by 2050.

Previous studies have only followed coral through one
bleaching event, or through two events several years apart. So Grottoli and her
team tested what would happen if they subjected some common Caribbean corals to
bleaching for two years in a row.

Corals can supplement their diet by eating plankton, but
they get most of their energy from their symbiotic relationship with algae. The
algae get nutrients from the coral, and the corals get to siphon off sugars
that the algae produce in photosynthesis. Like humans, corals can store excess
energy as fat.

Two key survival strategies emerged in this study: the most
resilient corals stored up fat reserves in times of plenty, and were willing to
switch to a new dominant algal type in order to gather food in times of stress.
Corals that didn’t store fat or were stuck with their algal partner didn’t fare
as well.

And species that bounced back from one round of bleaching
didn’t necessarily bounce back a second time.

“We found that the research on single bleaching events is
misleading,” Grottoli said. “Species
that we think are resilient to temperature stress are actually susceptible and
vice versa when stressed annually.”

Grottoli and her colleagues tested three corals from Puerto
Morelos Reef National Park, off the coast of Mexico. Two years in a row, they
plucked samples of Porites
divaricata, Porites
astreoides, and Orbicella
faveolata—more commonly known as finger coral, mustard hill coral, and
boulder coral—from the ocean floor, and placed them in warm water tanks in an
outdoor lab until the corals bleached. Both times, the researchers returned the
corals to the ocean to let them recover. They measured several indicators of how well
the different species recovered, including the number and type of algae present
in the corals’ cells and remaining energy reserve.

The mustard hill coral kept lower fat reserves, and
partnered with only one algal species. It recovered from the first round of
bleaching but not the second. The boulder coral kept moderate fat reserves, but
partnered with six different algae and changed between dominant algal types
following each bleaching. It recovered from both rounds of bleaching, though
it’s growth slowed.

The real winner was the finger coral, which switched
completely from one algal partner type to another over the course of the study,
and had the largest fat reserves—47 percent higher than the boulder coral or
mustard hill coral. The finger coral was barely even affected by the second bleaching
and maintained a healthy growth rate.

The bottom line: as some species adapt to climate change and
others don’t, there will be less diversity in reefs, where all the different
sizes and shapes of coral provide
specialized habitats for fish and other creatures. Interactions among
hosts, symbionts, predators and prey would all change in a domino effect,
Grottoli said. Reefs would be more vulnerable to storms and disease in general.

It sounds like a bleak picture.

“We’re actually a bit optimistic, because we showed that
there’s acclimation in a one-year window, that it’s possible,” she said. “In
two of our three coral species, we have recovery in six weeks. The paths they
took to recovery are different, but they both got there.”

Coauthors on the study included Grottoli’s former graduate students
Stephen Levas, Verena Schoepf, and Justin Baumann; Ohio State research
associate Yohei Matsui; and Mark Warner of
the University of Delaware and his graduate
students Matthew Aschaffenburgand Michael McGinley.